Read-only memory (ROM) is one kind of semiconductor memories. As its name implies, the read-only memory can only read stored information, and can not erase and rewrite the stored information. Data stored by the ROM is stable, even in the absence of power, the stored data can not be lost.
The read-only memory contains a plurality of storage units arranged in an array and configured to store information. Currently, the most common read-only memory in a market is a mask read-only memory. The manufacturing of the mask read-only memory needs an extra mask plate to form the storage units configured to store information. Performing an ion implantation process by using the extra mask plate to form two kinds of storage units with different threshold voltages. The information of the storage units is read by an operating voltage ranged between two kinds of threshold voltages. For example, when reading the information, the storage unit with the threshold voltage below the operating voltage will be turned on to obtain an electrical signal. The storage unit with a threshold voltage higher than the operating voltage will be turned off and obtain no electrical signal. Thus the information of the mask read-only memory can be read according to the above mentioned method.
However, this kind of mask read-only memory needs extra mask plates to form two kinds of storage units with different threshold voltages, which increases the manufacturing cost of the read-only memory.